The present disclosure relates to semiconductor devices including an inductor element.
In recent years, there has been an increasing demand for high-frequency circuits which are each implemented as a one-chip silicon integrated circuit in order to reduce the sizes of mobile communication apparatuses, such as mobile telephones and the like. However, the high-frequency circuit needs to include, as components, a transistor, a resistor, and a capacitor, and in addition, an inductor element, such as a coil or a transformer. Therefore, a method has been developed for forming, on a semiconductor substrate, an inductor element as well as an integrated circuit including a transistor, a resistor, and the like.
There is a method for forming a conductive film made of aluminum or the like which is in the shape of a spiral or a winding, as an inductor element, on an insulating film formed on a surface of a semiconductor substrate. In such a structure, however, the semiconductor substrate is disposed adjacent to the inductor element, and therefore, an eddy current which reduces a change in magnetic flux occurring when a current is passed through the inductor element, occurs in the semiconductor substrate. In this case, it is known that because the inductor element interacts with the eddy current occurring in the semiconductor substrate, there is a loss in a characteristic of the inductor element included in the semiconductor device.
Specifically, if the strip-like conductive layer in the shape of a spiral or a winding is assumed to be the primary coil of a transformer, the semiconductor substrate itself containing an impurity, which has a low resistance value, acts as a short-circuited secondary coil in a high-frequency region. The presence of the secondary coil causes a significant loss, particularly in the high-frequency region. Therefore, a technique of reducing the eddy current in the semiconductor substrate has been proposed (see, for example, Japanese Patent Publication No. H07-183468 (hereinafter referred to as Patent Document 1)).
Patent Document 1 describes that a plurality of P-N junctions are formed in a surface of a silicon semiconductor substrate, and an eddy current can be reduced in depletion layers which are generated in the P-N junctions and from which carriers are removed. Specifically, the path of the eddy current in the semiconductor substrate surface is segmented by the depletion layers to reduce the eddy current. Patent Document 1 also mentions a conventional example in which the eddy current is reduced by a depletion layer formed in the substrate surface.
FIG. 13 is a schematic diagram showing a structure of a conventional inductor element formed in a semiconductor device.
As shown in FIG. 13, a conductive film 16 having a spiral shape is formed on a surface of the semiconductor substrate 10. N-type impurity regions 14 are formed in the P-type semiconductor substrate 10 in the vicinity of the surface thereof, extending in the depth direction of the semiconductor substrate 10. In other words, a plurality of P-N junctions are formed in the vicinity of the surface of the semiconductor substrate 10.
The spiral-shaped conductive film 16 is formed on an insulating film 12. One end 16a of the conductive film 16 is connected to an interconnect (not shown). The other end 16b of the conductive film 16 is connected to an underlying interconnect 18 formed in the insulating film 12. When a current is passed through the conductive film 16 from the end 16a to the end 16b in a direction indicated an arrow 22, magnetic flux 20 is generated in the spiral interconnect.
According to the structure of FIG. 13, a plurality of depletion layers are formed in a plurality of P-N junctions, and therefore, a large number of depletion layers are formed in the surface of the semiconductor substrate 10. As a result, it is possible to increase the resistance to an eddy current which is generated in the semiconductor substrate 10 by the magnetic flux 20 generated by the inductor element made of the conductive film 16, whereby the eddy current can be reduced. Therefore, a reduction in an inductance characteristic due to a loss in the power and a reduction in the inductance which are caused by the eddy current can be reduced.